Many current attempts at electrophysiological elucidation of cardiac arrhythmia mechanisms have centered around activation sequence mapping. This is most commonly performed with polarized unipolar or bipolar metal electrodes, which because of unstable DC baseline potentials, necessitate AC coupled amplification. However, an ideal non-polarizable unipolar electrode would offer unhindered exchange of charge, allowing for stable DC recordings of biological electrical activity. In addition to activation information, DC unipolar recordings enable quantitation of systolic and diastolic potentials, other low frequency phenomena of interest such as repolarization, as well as rapid recovery from such rapid extreme potential shifts such as defibrillation. Previous attempts to apply non-polarizable electrodes to transmural cardiac investigations required complex wick electrode techniques to prevent mechanical movement of the fluid-metal interface when chlorided silver wire was used. We have developed a technique to fabricate miniature sintered Ag/AgCl electrodes that are mounted at various locations on a 20 gauge stainless steel needle permitting stable transmural DC unipolar electrogram recordings in vivo. The electrodes are low noise, rugged, sterilizable and reusable and should prove useful in three-dimensional electrophysiological characterization of the heart.
An electrode is non-polarizable if, when connected to a biological voltage source, a readily reversible chemical reaction occurs within the electrode and there is unhindered ionic exchange. These conditions are best met by using a metal in a solution of its own salt. For biological work Zn/ZnCl2, Hg/HgCl and Ag/AgCl have proven best. Electrodes made of any pure metal all polarize easily and form high-pass filters, eliminating the possibility of directly coupled (DC) recordings. Since the beginning of the twentieth century the Ag/AgCl electrode has shown itself to be the most stable of the easily constructed electrodes.
The larger the surface area, in general, the more stable are the recordings obtained from an electrode. Ag/AgCl electrodes may be fabricated in four ways: (1) electroplating chloride ions onto a pure Ag base; (2) coating a pure Ag base with molten AgCl; (3) suspending powdered Ag and AgCl in an electrically conductive matrix (1); (4) compressing a fine powder of Ag and AgCl and then heating to slightly below the melting point of the AgCl (melting point of AgCl=455.degree. C., melting point of Ag=960.8.degree. C.) to fuse the particles only at their points of contact. This fourth process is called sintering. It has the singular advantage of maintaining a large effective surface area in a small physical size. It also ensures some degree of mechanical strength, and in the case of metals, electrical continuity throughout the sintered object.
Techniques for making large sintered Ag/AgCl electrodes for cardiac recordings are known. DC potentials obtained with non-polarizable electrodes have permitted elucidation of the mechanisms of the ST segment elevation on the ECG seen commonly with acute myocardial ischemia and mapping of the associated injury currents. These results have been corroborated with DC magnetic field recordings which do not rely on an electrode interface to translate the biologically generated signal carried by ions to one carried electronically by electrons.
Neither the electrophysiological alterations induced by myocardial ischemia nor the pattern of normal activation and repolarization are homogeneous throughout the thickness of the ventricles in either man or in canine models. These require transmural recording techniques. Previous investigators have developed elegant wick electrode recording techniques to physically remove the electrode from the beating heart to obtain stable DC baselines within .+-.1 mV over the duration of an experiment (1-2 hours) To apply such techniques to obtain transmural recordings from hundreds of sites in vivo simultaneously would present major methodological difficulties. To address this problem we have developed an easily implanted non-polarizable needle electrode containing multiple transmural recording sites based on sintered Ag/AgCl technology.